The present invention relates to an automatic hue (tint) correction circuit for correcting hue automatically in a television receiver. This invention is suitable, in particular, for use in a NTSC television receiver.
A hue correction circuit which adjusts hue by handling a hue (tint) volume manually is known. The following document discloses an IC (integrated circuit) of HA11436 which contains an automatic hue correction circuit capable of correcting hue automatically: "IC DATA HOT SERVICE ", NIPPON TECH DATA Corp., June 1981, pp 6-322. FIG. 1 is a block diagram of this integrated circuit. In this figure, a first band-pass amplifier 2 amplifies a composite color picture signal applied from a detection circuit (not shown) in the television receiver to an input terminal 1. A second band-pass amplifier 3 amplifies an output signal of the first band-pass amplifier 2 and separates a chroma (color) signal and a burst signal from an amplified video signal. An automatic chroma control (ACC) circuit 4 controls a gain of the first band-pass amplifier 2 in accordance with the burst signal at one of the two output terminals of the second band-pass amplifier 3. A phase-locked loop (PLL) circuit 7 which includes a phase comparator 5 and a voltage-controlled oscillator (VCO) 6 generates a local subcarrier wave (CW) signal which is synchronized to the burst signal. A hue (tint) adjustment circuit 9 which is used togehter with a tint volume 8 of a variable resistor, performs a hue adjustment by controlling the phase of the CW signal. A first phase shift circuit (PS) 10 shifts the phase of the output signal of the hue adjustment circuit 9 by .phi..sub.1 and generates a CW.sub.R-Y signal which is a subcarrier signal on a (R-Y) axis. A second phase shift circuit 11 shifts the phase of the output signal of the hue adjustment circuit 9 by .phi..sub.2 and generates a CW.sub.B-Y signal which is a subcarrier signal on a (B-Y) axis. A demodualtion circuit 12 demodulates a (R-Y) color difference signal by using the CW.sub.R-Y signal, and also demodulates a (B-Y) color difference signal by using the CW.sub.B-Y signal. Further, the demodulation circuit 12 forms a (G-Y) color difference signal by matrixing the (R-Y) signal and the (B- Y) signal. Therefore, the circuit thus arranged can produce the (R-Y) color difference signal, the (G-Y) color difference signal and the (B-Y) color difference signal with the composite color picture signal applied to the input terminal 1.
Furthermore, the circuit structure of FIG. 1 has the following elements. A third phase shift circuit 16 generates a CW.sub.Q which is a subcarrier signal on a Q axis shifted by .phi..sub.3 relative to the CW signal. A fourth phase shift circuit 17 generates a CW.sub.I signal which is a subcarrier signal on an I axis shifted by .phi..sub.4 relative to the CW signal. A Q axis-demodulation circuit 18 generates a Q signal by demodulating the chroma signal on the Q axis by the CW.sub.Q signal. An I axis-demodulation circuit 19 generates an I signal by performing the I axis-demodulation of the chroma signal by the CW.sub.I signal. An amplification circuit 20 amplifies the Q signal derived from the Q axis demodulation circuit 18 and adjusts the hue adjustment circuit 9 by an amplified Q signal. A control circuit (CONT) 21 compares the I signal derived from the I axis demodulation circuit 19 with a reference signal (V.sub.ref), and makes the amplification circuit 20 operate when the I signal exceeds the reference signal (V.sub.ref), and on the contrary makes the amplification circuit 20 non-operable when the I signal does not exceed the reference signal. Therefore, in the circuit thus arranged, when the chroma signal approaches the I axis and then its level exceeds that of the reference signal, the amplification circuit 20 is driven by the control circuit and the hue adjustment circuit 9 adjusts hue so that the chroma signal near the I axis overlaps with the I axis by using the amplified Q signal. A signal on the I axis corresponds to flesh color. Therefore, the hue adjusting described previously means that a color near that of flesh color is corrected so as to be changed into the correct color.
The phase relationship between the CW signal, the CW.sub.R-Y signal, the CW.sub.B-Y signal, the Q signal and the I signal is shown in FIG. 2.
As apparent from the foregoing, the prior circuit is so designed that the automatic hue correction is performed in accordance with the level of the Q signal formed from the chroma signal by the Q axis-demodulation circuit 18.
However, the prior automatic hue correction circuit of FIG. 1 has the following disadvantages. The circuit has no feedback loop so that a corrected result, or the output signal of the hue adjustmen circuit 9 is returned to the input side of the hue correction circuit. Thus, the following two operative conditions are, in fact, required to carry out a precise hue correction. First of all, the gain of the amplification circuit 20 is an important factor. That is, the gain affects the hue correction. In details, the compensation of hue will excessively performed if the gain is too large and, on the other hand, the compensation will be insufficient if the gain is too small. This means that it is necessary to adjust the gain of the amplification circuit 20 strictly, which causes a complicated adjustment operation of the gain. Secondly, it is required that the adjustment characteristic of the hue adjustment circuit 9 is identical with the characteristic of the Q signal. However, it is difficult to obtain this identity.